Rashi: Yenachashu is an expression of Nichush. They said, since he said words of Shalom, even if he did not intend for this, it is a good Siman. They rushed to strengthen the matter, lest he retract.

Radak: They did Nichush; if Achav will give a nice answer, it is a sign that he will keep him alive. "Va'Ymaharu va'Yachletu" are both expressions of rushing. Before Achav finished saying "Achi Hu", they said "Achicha." Doubling like this strengthens the matter, e.g. "ha'Akeh Akeh Avi" (Melachim II, 6:3), "ha'Idarosh Idaresh Lahem" (Yechezkel 14:3), "ba'Derech ba'Derech Elech" (Devarim 2:27) and similar verses.

Malbim: Initially they did not believe that Achav said so sincerely. They began to do Nichush to determine this, but immediately decided that it was sincere and called him Achicha, so Achav could not retract.

Rashi: We put the prefix Hei at the end of the previous word "va'Yachletu", as if it says va'Yachletuha. They resolved that the words came from him with intent. Radak - Targum Yonasan is like this.

Radak: It refers to Yenachashu (they did Nichush based on Achav's response).

Radak citing Targum Yonasan: Go up (enter the city) and take him [to me].